In order that a plastics material product container might be adopted as an acceptable substitute for a metal container, the plastics material product should meet certain criteria. It should be reusable, in the sense that the closure should be capable of resealing the container portion following successive openings. The primary seal between the closure and the container portions should be made internally of the container wall, in order that the container might be dimensioned for compatibility with conventional labelling machinery. The closure should provide an hermetic seal with the container portion but be readily removable from the container portion with use of a prising implement such as a screwdriver blade. Finally, the container-closure arrangement should be capable of absorbing the energy of normally encountered impact forces without the closure being caused to break away from the container portion.
Various prior art product containers have been developed in an attempt to meet the above stated criteria but, to the knowledge of the present inventor, no prior art container has met the seemingly incompatible requirements for a satisfactory impact resistant container-closure seal and convenient removability of the closure from the container. The inventor has found that, in practice, some prior art containers which have been developed for packing liquid materials such as paint have met the need for relatively easy removal of the closure from the container by providing a small interference seal between the closure and container portions or by forming the closure from very soft deformable materials such as low density polyethylene, but, as a consequence, such arrangements exhibit a poor resistance to impact forces.
Of all the prior art product containers of which the inventor is aware, it is thought that those disclosed by U.S. Pat. No. 3,977,563 and U.K. Pat. No. 1,388,331 are the most relevant in the context of the present invention. However, for reasons which will become clear from further reading of this specification, it is sufficient to say that, in the case of both of the referenced patents, the disclosed prior art closures do not incorporate wall portions which have a reduced thickness in a region which is strategically located for the purpose of elastically absorbing impact forces, and, at the same time, causing increased sealing between the closure and container portions.